The most common optical waveguide is the fiber with a round-shaped core supported by a round-shaped cladding. The next common optical waveguide is a planarized waveguide in which light-guiding channels are fabricated near the surface of an usually right-angled optical substrate. In the planarized waveguide the light-guiding core is often called optical channel waveguide. Many of the useful optical functions, such as light modulation, wavelength multiplexing, switching, and coupling may be realized on planarized waveguides.
Guided light resides mostly inside the core or the channel. The sectional dimension of the fiber core is typically less than ten microns (0.01 mm) in single-mode fibers, and usually less than 200 microns even in the multimode fibers. The sectional dimension for channel waveguide is in the same range. Accordingly, connecting and coupling (mixing) of light between two or more fibers present enormous technical challenge. As a result, the prices for connectors and couplers for optical fibers are quite expensive, especially when compared to the counterpart components for microwave cables. Since the connectors and the couplers are among the most frequently used components in the optical fiber communication, the high component price has impeded the expansion of the optical fiber communication into the broader applications, such as picture-phone, computer networking, and cable television.
The parent patent referred to above has been dealing mainly with the fiber couplers for dividing and combining lights among many optical fibers laid in parallel. The present continuation-in-part application instead deals with optical fiber connectors for transferring a light from one optical fiber to another optical fiber in one-to-one relationship, and also between an optical fiber and a channel waveguide in the one-to-one relationship.
Optical fiber connector is one of the key components in fiber optics, especially in the optical fiber communication. The cost becomes the critical issue when the applications come close to end-users, such as interconnecting computer networks. The existing optical fiber connectors are very expensive and intricate for such applications. Also such applications will require multi-fiber array connectors, the counterpart of multipin connectors for electronic cables. Array connectors minimize connector space, per-connection cost, and overall connection time. Technology for such multi-fiber array connectors are in its infancy at best at the present time, and the price is impractically high.
In general, the connection between fibers becomes easier when light beam is enlarged in size in the mating plane. When the beam is enlarged, the alignment tolerance becomes relaxed, while the angular tolerance becomes more stringent. For example, Wasserman and Gibolar show in U.S. Pat. No. 5,097,524 a connector embodiment that employs lenses to expand light beam. Moslehi et. al. describes in Optics Letters, Volume 14, Number 23, on page 1327, a fiber optic connection based on expanded-beam optics. Hussey and Payne describes in Electronics Letters, Volume 24, Number 1, on page 14, a fiber-horn beam expander. However these techniques still require critical alignment between fibers and the beam-expanding elements. Also, these prior arts are for single fiber connection, and do not lend themselves to array connection.
Another important fiber optic technology is connection between an optical fiber and a channel waveguide. Currently, channel waveguides are patterned on or near the fiat top surface of a bulk optical substrate using photo-lithography or other advanced techniques such as electron-beam or laser-beam writing. In most of the applications, a channel waveguide needs to be connected to an optical fiber in one-to-one, end-butt fashion. To make this connection, the end of the channel waveguide should be cut flat and right-angled with respect to the waveguide plane, and then polished with the fabrication tolerance in the order of a fraction of the optical wavelength while keeping the edge sharply right-angled within one or two microns from the substrate surface. Then an optical fiber with a cleaved facet is brought against the such-prepared end facet of the channel waveguide. The lateral alignment between the optical fiber core and the channel waveguide should be made within a few microns or less. Then a cementing material is applied to the butted region. The alignment often deteriorates while the cement is being cured due to the volume change and shift, causing connector loss. Even with the perfect alignment, the shape mismatch between the round fiber core and the largely square-shaped channel waveguide causes substantial connector loss. Overall, a fiber-to-channel connection is a very expensive fabrication step. This is another reason why the fiber optics has not been able to penetrate into the wider consumer market despite of the enormous potential benefits.